Opto-chemical and electrochemical sensors frequently comprise measuring membranes that are put in contact with a measuring fluid, e.g., a measuring gas or a measuring liquid, in order to capture measured values. The measuring membranes have at least a sensor-specific function layer that has different functions, depending upon whether the sensor is an opto-chemical or an electrochemical sensor.
Many electrochemical, especially amperometric, sensors have an electrolyte chamber separated from the measuring fluid by means of a measuring membrane. The measuring membrane in amperometric sensors for determining a gas concentration in a liquid, e.g., electrochemical O2, Cl2, CO2, H2S, NH3 or SO2 sensors, comprises at least a function layer acting as a diffusion barrier letting the analyte diffuse from the measuring fluid into the electrolyte chamber. Such a sensor is described in, for example, DE 10 2008 039465 A1.
An opto-chemical sensor, e.g., an opto-chemical oxygen, ozone, or carbon dioxide sensor, may be based upon the principle of analyte-induced fluorescence or luminescence quenching of an organic dye—a so-called fluorophore. Opto-chemical sensors frequently comprise a sensor element featuring the measuring membrane. The sensor-specific function layer of the measuring membrane comprises the fluorophore for opto-chemical sensors. The function layer may be designed, for example, as a polymer layer in which the fluorophore is dissolved. The polymer layer is brought into contact with the measuring fluid to capture measured values. Due to interaction of the fluorophore with the analyte, the fluorescence and/or luminescence intensity of the fluorophore as a function of the analyte concentration in the measuring fluid decreases. Usually, the measuring membrane is applied to a substrate, e.g., to a glass plate or an optical fiber, to create a sensor spot.
From WO 2005/100 957 A1, we know about an opto-chemical apparatus for determining and/or monitoring an analyte contained in a fluid process medium. The known apparatus has a sensor with a measuring membrane having a porous carrier structure. A luminescent substance coming into contact with the measuring medium is embedded into the carrier structure. Furthermore, a sender and a receiver are provided, with the sender emitting measuring beams and stimulating the luminescent substance to emit luminescence radiation, and the receiver detecting the respective generated luminescence radiation. A control/evaluation unit determines the concentration and/or the partial pressure of the analyte in the measuring fluid on the basis of the quenching of the luminescence radiation of the luminescent substance.
From DE 100 51 220 A1, we know about an optical sensor for determining an analyte, especially oxygen, that mainly shows a sensor matrix consisting of a fluoro-polymer. The sensor matrix contains a luminescence indicator containing a metal complex of ruthenium, rhenium, rhodium, or iridium and at least a partially fluorinated ligand. The sensor matrix itself is designed as a film and equipped with a protective layer. The protective layer is preferably made of the same material as the sensor matrix, but does not include a luminescence indicator. Any mechanical damage to the sensor matrix is counteracted by the protective layer.
DE 10 2014 112 972 A1 describes a measuring membrane for an opto-chemical or electrochemical sensor. The measuring membrane comprises a sensor element that features at least one function layer with a sensor-specific substance and a substance material, wherein the sensor element is fully embedded into a matrix and wherein the matrix consists of a material that is accessible for the analyte at least in a partial area facing the medium and adjacent to the sensor element. The measuring membrane may be housed in a cylindrical sensor cap that is exchangeably connected with a probe body of the sensor.
In processes in the food industry or in bio-chemical or bio-technological processes, foam frequently occurs, caused by the presence of, for example, proteins. However, cleaning and disinfection procedures may also be disturbed by undesired foaming.
When the sensor is vertically installed and the measuring fluid contacting the sensor moves only with limitation, e.g., at a low stirring speed, there is frequently a problem of bubbles forming, or the accumulation of bubbles or foam on the measuring membrane. Gas bubbles clinging to the membrane may corrupt the measured values captured by the sensor. The disappearance of a gas bubble attached to a measuring membrane may, depending upon the design of the sensor, especially the above-mentioned sensor cap, take some minutes or hours.
There are various procedures aimed at preventing or suppressing the creation of foam. One option is mechanical foam destruction. A method for mechanical foam destruction is described in EP 35705 B1, in which foam is removed by a turning intake socket. This solution certainly contributes to improvement. However, since it is mostly impossible to achieve complete foam prevention, it is advisable to arrange functions directly on the sensor that prevent any foam from attaching itself to the sensor.
Often, the approach is chosen of adding substances to the measuring fluid that suppress foam generation or are meant to destroy any foam generated. However, this is not always feasible, especially if the additives might disturb the process that is to be monitored by means of the sensors.
Another option for dealing with gas bubbles that interfere with the measurement is described in U.S. Pat. No. 6,914,677 B2. In this case, it is a sensor that discovers bubbles on the sensor by detection via a second light channel. However, bubble generation is not prevented with this method.
One solution with a physical approach for preventing gas bubbles is achieved with a turbidity sensor. DE 10 2013 111416 A1 describes a turbidity sensor with an ultrasonic unit that ensures that the sensor remains bubble-free. Despite the fact that this method satisfactorily prevents the accumulation of irritating bubbles, it has the disadvantage that operating the ultrasonic unit requires additional energy, which is not always readily available for opto-chemical or electrochemical sensors.